The life cycle of Toxoplasma gondii has two phases. The sexual part of
the life cycle takes place only in members of the Felidae family
(domestic and wild cats), which makes these animals the parasite's primary
host. The asexual part of the life cycle can take place in any
warm-blooded animal, like other mammals (including felines) and birds.
T. gondii constructing daughter scaffolds within the mother cell.In the
intermediate non-Felidae hosts, the parasite invades cells, forming
intracellular so-called parasitophorous vacuoles containing bradyzoites,
the slowly replicating form of the parasite[1]. Vacuoles form tissue
cysts mainly within the muscles and brain. Since they are within cells,
the host's immune system does not detect these cysts. Resistance to
antibiotics varies, but the cysts are very difficult to eradicate entirely.
Within these vacuoles T. gondii propagates by a series of binary
fissions until the infected cell eventually bursts and tachyzoites are
released. Tachyzoites are the motile, asexually reproducing form of the
parasite. Unlike the bradyzoites, the free tachyzoites are usually
efficiently cleared by the host's immune response, although some manage to
infect cells and form bradyzoites, thus maintaining the infection.
Tissue cysts are ingested by a cat (e.g., by feeding on an infected
mouse). The cysts survive passage through the stomach of the cat and the
parasites infect epithelial cells of the small intestine where they
undergo sexual reproduction and oocyst formation. Oocysts are shed with the
feces. Animals and humans that ingest oocysts (e.g., by eating unwashed
vegetables etc.) or tissue cysts in improperly cooked meat become
infected. The parasite enters macrophages in the intestinal lining and is
distributed via the blood stream throughout the body.
Acute stage toxoplasma infections can be asymptomatic, but often gives
flu-like symptoms in the early acute stages, and like flu can become,
in very rare cases, fatal. The acute stage fades in a few days to
months, leading to the latent stage. Latent infection is normally
asymptomatic; however, in the case of immunocompromised patients (such as those
infected with HIV or transplant recipients on immunosuppressive therapy),
toxoplasmosis can develop. The most notable manifestation of
toxoplasmosis in immunocompromised patients is toxoplasmic encephalitis, which
can be deadly. If infection with T. gondii occurs for the first time
during pregnancy, the parasite can cross the placenta, possibly leading to
hydrocephalus, intracranial calcification, and chorioretinitis, with
the possibility of spontaneous abortion or intrauterine death.
